Ever since the introduction of fiber optics technology, the optical fiber has proven to be a versatile and relatively efficient means of transporting light energy and information. For example, optical fibers are used in the medical field to transport laser energy through flexible catheters for pin-point microsurgery, or in the telecommunications field to transport data for long distances at very high rates. More recently, developments in fiber optics technology have spurred considerable research on the use of optical fibers as pressure or strain sensors.
The need for improved pressure sensors has been widely recognized in the research and development community. For example, spring-activated pressure gauges have been used in the automotive industry to measure dynamic pressures in automobile and truck engines. In one application, a port in a cylinder wall or head of an engine is provided for pressure measurements. Gas exiting the combustion chamber through the port passes through a metal tube to the spring-activated gauge. However, spring-activated pressure sensing devices do not operate reliably at high temperatures. Consequently, the device must be located remotely from the combustion chamber, which reduces the response time of the sensor and introduces measurement errors due to acoustical effects and pressure drops in the elongated tube. The relatively slow response of such sensors limits their utility to average pressure measurements, and the resulting measurement errors reduce the accuracy and reliability of the measurement data.
More accurate pressure readings, with acceptable time resolutions, have been provided by using piezoelectric transducers as sensors. In a conventional application, a piezoelectric transducer is mounted in a special port, typically in the cylinder head of an engine. Two types of such transducers may be used. In one type, the piezoelectric element is exposed directly to the pressure in the combustion chamber. In the other type of transducer, a piston is located between the chamber and the piezoelectric element. Consequently, the element responds to the displacement of the piston and only indirectly to the pressure in the chamber. A disadvantage of piezoelectric transducers is that they must be cooled with recirculating water or air because of their vulnerability to the high temperature environment in which they must operate. Nevertheless, even with optimum cooling, piezoelectric transducers must be frequently recalibrated, their operating life is relatively short, and the sensors are relatively expensive. Generally, piezoelectric pressure sensors have proven useful for engine research and development, but due to their limitations, they have not been widely incorporated into engine products.
Recent advances in embedding optical fibers in metals have spurred the development of fiber optic pressure sensors. For example, commonly-assigned U.S. patent application Ser. No 07/926,756 (Attorney Docket No. 17575-0135) describes a novel technique for embedding optical fibers in metals, by using stress-relieving tubes at the air-metal interfaces of the fibers. Previous attempts to embed optical fibers in metals had all resulted in failure. Even more particularly, however, Vol. 16, No. 24 of Optics Letters., Dec. 15, 1991, pp. 1990-92, describes the sensing of temperature and ultrasonic pressure with fiber optic Fabry-Perot interferometers embedded in aluminum parts. Breakage of the fibers at the air-metal interface during the embedding process is avoided through the use of stainless-steel stress-relief tubes. However, these fibers are fed completely through the aluminum part, which may be useful for monitoring temperature and strain in aluminum structures such as aircraft wings, but prohibitive in applications where the sensor must not be intrusive, such as, for example, monitoring pressure inside the combustion chamber of an engine. Therefore, it follows that if a non-intrusive, metal-embedded fiber optic pressure sensor were possible, then the problems encountered with spring-activated gauges and piezoelectric pressure sensors would be overcome.